In recent years, much attention has been focused on the reduction of particulate matter and nitrogen oxides exhausted from automotive diesel engines such as those on buses and trucks. Similarly, the removal of harmful substances contained in the exhaust gas of engines such as diesel engines for ships and power generators, boiler exhaust gas, and plant off-gas is an important problem. However, while automotive diesel engines use diesel fuel oil for fuel, which has a low sulfur content, diesel engines such as those on ships and power generators use high sulfur content fuel known as Class A or Class C fuel oils (JIS standard). For this reason, the exhaust gas of the latter contains a large amount of sulfur oxides, and processing the gas to remove harmful substances is a significant obstacle.
Typically, non-catalytic denitration and selective catalytic reduction (SCR) are known as denitration processes for exhaust gas. Non-catalytic denitration is widely known as a denitration process using a nitrogen-based reducing agent such as ammonia or urea, but unless the exhaust gas is in a high-temperature state (900° C. to 1000° C.), high-activity catalysis cannot be obtained (for example, see the patent literature 1 and 2). For this reason, given the relatively low-temperature (approx. 250° C. to 450° C.) exhaust gas exhausted from diesel engines such as those on ships and power generators, it is necessary to pre-treat the exhaust gas to heat it and raise the temperature thereof, which in turn leads to increased treatment costs and makes the application of non-catalytic denitration problematic.
In addition, while non-catalytic denitration using ammonia achieves a high denitration rate at the laboratory level, it has been difficult to obtain a denitration rate exceeding 50% in an actual furnace such as a boiler. For example, the non-patent literature 1 shows a denitration process wherein, for a temperature range of 900° C. to 1050° C., the supplied quantity of ammonia is divided into a pre- and a post-stage, and these ammonia quantities are respectively controlled and supplied according to the boiler load. However, the denitration rate of this process is only approximately 40% under equimolar conditions of nitrogen oxides and ammonia. Furthermore, if ammonia is excessively supplied in order to raise the denitration rate, unreacted ammonia remains and treatment costs increase. Moreover, in the case where the exhaust gas contains sulfur oxides, ammonium sulfate is created, so that the treatment thereof leads to worsened cost-effectiveness.
In contrast, the patent literature 3 proposes as an SCR process a method in which nitrogen monoxide in the exhaust gas is oxidized to nitrogen dioxide; subsequently, in the presence of an SCR catalyst, a reducing agent of ammonia, urea, a hydrocarbon, or the like is added; and the nitrogen dioxide is catalytically reduced. However, this SCR process is inferior to the non-catalytic denitration process in that it uses a large quantity of SCR catalyst. Furthermore, there is the problem that when the exhaust gas temperature is 300° C. or less, sulfur dioxide in the exhaust gas that has oxidized to sulfur trioxide and the like reacts with the ammonia to create ammonium hydrogen sulfate, which poisons the SCR catalyst and reduces catalytic activity. For this reason, the SCR process is only applied when exhaust gas is in a high-temperature state of 300° C. or greater whereby the ammonium hydrogen sulfate decomposes, or when the concentration of sulfur oxides in the exhaust gas is approx. 1 ppm or less.
In such circumstances, the patent literature 4 proposes a method in which a heating zone is formed in a flue or in a chamber communicated with the flue, the flue carrying a low-temperature exhaust gas containing sulfur oxides; subsequently, nitrogen compounds and hydrocarbons are blown toward this heating zone to form amine radicals; and these amine radicals denitrate the nitrogen oxides in the exhaust gas. However, the denitration rate in this denitration process is not entirely sufficient, and there is demand to further raise the denitration rate.    Patent Literature 1: U.S. Pat. No. 6,066,303    Patent Literature 2: Japanese patent application Kokai publication No. 2002-136837    Patent Literature 3: Japanese patent application Tokuhyo publication No. 2001-525902    Patent Literature 4: Japanese patent application Kokai publication No. 2005-254093    Non-patent Literature 1: “Fuel Conversion and SOX/NOX Countermeasure Technologies: A Focus on Exhaust Desulfurization/Denitration”, by Junpei Ando (Project News, Jun. 25, 1983, p. 205-207)